The present invention relates to communication systems for the transmission of information in secure form and more particularly to such systems in which the information signal is coded by a transmitter and decoded by the receiver at the respective transmit and receive location, the communication between the locations being by any suitable means such as telephone, radio, or other communication systems.
Heretofore, systems have been proposed by which the transmission of information signals may be rendered secure and difficult to decode. Among the proposals heretofore made have included the use of pseudorandom digital generators at the receive and transmit locations, which generators may be caused to operate in synchronization and on the same pseudorandom code base so that the receiver and transmitter can be locked together. Several proposals have been made for ways in which this can be implemented. More specifically, spread spectrum techniques have been proposed and are summarized, for example, in the book by R. C. Dixon entitled Spread Spectrum Systems published by John Wiley & Sons, New York, 1976.
In a typical spread spectrum system a base band signal, say a voice channel, of perhaps only a few kilohertz, is distributed over a band of carrier frequencies that may be many MHZ wide. Often the spread spectrum technique leads to modulation of a carrier by frequency shifting or frequency hopping techniques according to a pre-determined pseudorandom code or pulsed FM chirp modulation in which the carrier is swept over a wide band during a given pulse interval. In general, the systems as used in spread spectrum technology require complex transmitting and receiving equipment capable of wide band operation and also require expensive frequency selective apparatus for implementation.
Other prior art systems have been proposed utilizing pseudorandom signal generators for scrambling or encoding the information signal itself for secure transmission rather than the carrier system employed in the transmission. Reference is made, for example, to U.S. Pat. No. 3,909,534 entitled VOICE PRIVACY UNIT FOR INTERCOMMUNICATION SYSTEMS, issued Sept. 30, 1975 to Henry L. Majeau, et al. In that patent it is proposed that a pseudorandom generator be used to control a frequency shift modulation device which is applied to the information signal being processed. In this way, the pseudorandom code selects from one of the modulation frequencies available and is time duration coded by them in a pseudorandom way, which resulting coded frequency pattern is mixed with the information signal before being transmitted. In the 3,909,534 patent there is required at the receiver a pseudorandom code generator controlled by an independent frequency synthesizer and suitably synchronized to the transmitter for providing a selected modulation frequency code which corresponds to that transmitted. Disadvantages to this system include the necessity for modulating the information signal with signals of varying frequency and the entire use of multi-frequency encoding according to the pseudorandom code may render the transmission less secure. Furthermore, the complexity of the transmitter and the receiver is unduly high. In addition, the synchronization of such equipment relies both on right initiation of the initial sync pulse generator and the inherent reliability and quality of the frequency generators of the transmitter and receiver.
Another system as disclosed in the U.S. Pat. No. 3,614,316 illustrates the use of direct sequence system requiring two pseudorandom generators. In U.S. Pat. No. 3,614,316 the information signal is used to phase modulate one of the pseudorandom generators which is then operated to transmit the sum of the two generators as the encoded information. Obviously, the requirement of the use of two pseudorandom generators results in a complex and costly system. There is, therefore, a need for a new and improved communication system which will provide for secure communications between transmit and receive locations.